System powered damper and control units of the general type under consideration have heretofore been available as illustrated in U.S. Pat. No. 3,877,452, June 18, 1974, entitled DUCT PRESSURE ACTUATED VARIABLE VOLUME DEVICE. While such units have been generally satisfactory in operation, certain disadvantages have been encountered. Actuating bellows, disposed within an air conduit, housing, etc., and exposed externally to conduit air require an increment of operating pressure to overcome the externally acting air pressure and, in addition, such bellows must meet fire code regulations as regards contribution to smoke, fuel, etc. Further, a uniform and tight closing operation of the damper has not always been provided for with the result that cooling or heating air is unnecessarily lost to the detriment of energy conservation considerations.